It is well known in the art that a laser may be used to amplify optical signals, provided only that the threshold of laser oscillations is not exceeded. That is, a laser may be used as an amplifier when the product of the total gain of light signals passing through the optical resonator of a laser and the mean reflection coefficients of the resonator end mirrors is less than unity. When such product is greater than unity typical laser oscillations occur, the frequency of such oscillations being dependent upon the particular laser being used.
It has been conventional practice to use a particular laser as either an amplifier or an oscillator. When a laser is used as an oscillator the dimensions of its optical resonator are controlled, by one means or another, to make such resonator a high Q cavity wherein regenerative amplification occurs for light waves of a given wavelength. The Fabry-Perot interferometer incorporating a pair of mirrors, properly aligned and spaced one from the other, is the classical example of an optical resonator. A laser amplifier, however, requires only an optical cavity filled with a medium having an inverted population. As a result, any signal within a proper frequency band may initiate stimulated emission to produce an amplified optical signal.
It is also known to provide an arrangement of lenses and mirrors to separate a beam of coherent light transmitted by a laser from the light backscattered from illuminated targets so that optical heterodyning of such backscattered light may be accomplished. In known arrangements of such type, great care must be taken to maintain the alignment of the various lenses and mirrors to attain satisfactory results. As a matter of fact, the alignment problem is so critical that, except under controlled laboratory conditions, environmental problems, such as vibration, make it extremely dificult to design and make satisfactory systems.